The present invention relates to an improved process for making ceramic articles from extrusion or injection moldable ceramic compositions. More particularly, the present invention is directed to a process for binder removal from a molded ceramic greenbody containing ceramic powder and a polyacetal binder.
Ceramic materials are of critical importance for a number of high temperature, high performance applications. Recently, there has been substantial interest in the development of ceramic compositions for critical engine parts including reciprocating engines, gas turbine and rocket engines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. However, the inability to produce complex shapes of high dimensional accuracy and sufficient strength using an economical fabrication technique has prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
Several processes have been used in an attempt to form ceramic bodies. Among such processes include pressing ceramic powder into a greenbody followed by sintering or by hot pressing and subsequently shaping or machining the sintered body to produce the finished product. Another technique is slip casting in which the ceramic particles are dispersed in water, the slurry placed in a mold and the water removed to form a greenbody. The pressing techniques have been found unsuitable to form ceramic articles of complex shapes and which must meet specific design specifications. The slip casting technique is limited since it allows for only parts which have uniform thickness.
In view of the problems associated with the prior techniques, injection molding has been increasingly used to form ceramic articles. Injection molding is a process wherein a moldable composition is forced into a mold or die. The injection molding process facilitates a rapid and repeated forming of a plurality of articles having a consistency with close dimensional tolerance. The injection molding process also minimizes the amount of shaping or machining that may be required to produce a finished article.
The whole injection molding process to form a ceramic article typically involves forming a ceramic greenbody by injection molding a composition comprising ceramic powder dispersed within a thermoplastic polymer, burning out the polymer, and sintering the resulting porous greenbody to a dense ceramic part with the same shape. The thermoplastic binder acts as a fluidizing agent to distribute the injection pressure throughout the mold and as the material which holds the ceramic particles in the shape of the mold after the part is ejected. A typical ceramic powder/thermoplastic polymer composite has a very high content of ceramic particles, typically from about 50 to about 87 volume % and a minimum of binder material to hold the particles together in the desired shape. A useful binder material for ceramic injection molding is a polyacetal resin as disclosed in U.S. Pat. No. 4,624,812, the entire contents of which are herein incorporated by reference.
A typical injection moldable ceramic composition will also contain a minor binder component which is often a thermoplastic, wax or oil, plasticizers which increase the fluidity of the ceramic-binder mixture, and processing aids such as surfactants which improve the wetting characteristics between the plastic binder and ceramic during mixing to form the composite.
A summary of injection molding applied to the fabrication of molded ceramic bodies is provided in an article entitled "Review: Fabrication of Engineering Ceramics by Injection Molding. I. Materials Selection", M. J. Edirisinghe et al, International Journal of High Technology Ceramics, Vol. II, 1986, pp. 1-31.
A major problem with regard to the formation of ceramic articles by injection molding relates to the difficulty of removing the binder such as by a thermal degradation process. Often, during binder burn-out, the generation of internal pressures from gaseous decomposition products disrupts the integrity of the greenbody such as in the formation of cracks, blisters and other surface deformities. To reduce the effects of the escaping gaseous decomposition products from the binder, it is often necessary to utilize an extremely flat temperature profile during the burn-out stage. Unfortunately, a slow rise in temperature to achieve binder burn-out results in an excessively long time, i.e., greater than 10 days and even greater than 20 days, before the binder is removed, thereby greatly reducing the commercial viability of the injection molding process to form ceramic articles.
Also, the rate of extraction of the binder depends on the thickness of the greenbody in an inverse relationship, i.e., the thicker the cross section of the greenbody, the longer it takes to extract the binder. In general, the rate of binder extraction is limited as a result of the generation of gaseous decomposition products which tend to disrupt the integrity of the greenbody and reduced extraction efficiency as degradation products build up within the greenbody. The result is often the introduction of practical constraints for part wall thickness in order to keep the process economical as well as the use of very flat temperature profiles which as discussed previously greatly lengthens the burn-out process and, thus, reduces the economic viability of the injection molding process for producing ceramic articles.
An alternative process for removing the binder is to extract the binder from the molded part with a solvent either in liquid or gaseous form. Solvent extraction of the binder, for example, is disclosed in U.S. Pat. No. 2,939,199. Typical solvents utilized to extract binders from ceramic greenbodies include acetone, methylethyl ketone, carbon tetrachloride, trichloroethylene, methylene chloride, etc. However, it is known that when the binder and solvent are mixed together, the binder-solvent mixture has a tendency to expand. If the binder is the solid state, it cannot readily move within the greenbody and the expansion within the greenbody can become greater than the forces holding the particulate material together. The result is often cracking which is further accentuated in the final firing step. Attempts have been made to reduce the cracking which occurs in the molded ceramic greenbodies when using solvent extraction as the binder removal process. For example, U.S. Pat. No. 4,197,118 discloses a process of removing the binder by a gaseous solvent in which the greenbody is heated to allow the binder to melt and wherein the gaseous solvent is then contacted with the greenbody at that point. While the patent states that such process reduces the swelling forces of the solvent and binder, the solvent is still dissolving the binder as a whole. Accordingly, the bulky binder component must still be passed through the interstices of the greenbody. It is very difficult to remove the binder as a bulk molecule from the ceramic greenbody without causing disruptions in the greenbody itself as the bulk compound passes through the pores of the greenbody. To eliminate cracking, the process has to be run at a slow rate of contact with the solvent, thus, again vastly reducing the economic gain achieved by the injection molding process.
While the use of polyacetal binding agents as disclosed in the aforementioned U.S. Pat. No. 4,624,812 is advantageous since the polyacetal depolymerizes relatively easily during the burn-out process, sharp temperature profiles tend to cause disruptions on the article's surface in the form of cracks and blisters. The use of a flat temperature profile, on the other hand, results in excessive time to completely remove the binder. Such time can be in excess of 50 hours. Accordingly, the thermal degradation of the polyacetal binder from the ceramic greenbody is difficult to accomplish in an economical manner.
It is therefore an object of this invention to provide an improved process for forming ceramic articles by the injection molding process. A further object of this invention is to provide an improved process for removing the organic binder from a molded ceramic greenbody in which a polyacetal resin is used as the binder. It is still a further object of this invention to provide an improved process for removing a polyacetal binder from a molded ceramic greenbody without disrupting the surface of the formed ceramic article and which binder removal process can be achieved in a substantially shorter time than previously contemplated.